They were going to lose customers to Airbus, who had a longer range plane. The solution was to retrofit larger engines on the 737. But they were too large and

They were going to lose customers to Airbus, who had a longer range plane. The solution was to retrofit larger engines on the 737. But they were too large and collided with the ground. They moved the engines to the front of the wing for ground clearance. This made the plane unstable and tend to tilt upwards, so they came up with MCAS to work around the instability. The plane is not airworthy.